In recent years, traditional radio-navigational systems, such as Loran (long range navigation), have been slowly replaced or relegated to a backup role for more accurate satellite navigational systems, such as GPS (global positioning system). However, complete replacement of radio-navigational systems with GPS has not occurred thus far, due to some of the shortcomings of GPS.
In fact, there has been a renewed interest in systems such as Loran to provide backup for GPS systems, in the event of failure of the GPS systems. Accordingly, Loran systems are being studied to determine whether they can be updated to provide a reliable backup system for GPS.
Loran antennas used in most transmission sites are characterized by having relatively short electrical lengths compared to the transmitted wavelength. The antenna, highly capacitive due to the short electrical length, is normally series resonated with a loading inductor to minimize the reactance at the center frequency. The resulting tuned circuit has a very narrow bandwidth with a quality factor (Q) typically in the range of 20 to 60.
The ideal transmitted Loran signal has a bandwidth that considerably exceeds the bandwidth of the transmission antenna. It is normally a requirement for any transmitter system that the bandwidth of the antenna exceeds the bandwidth of the transmitted signal. In the case of Loran, the antenna bandwidth deficit makes the antenna unsuitable for a typical transmitter, such as a Long Wave or Medium Wave Amplitude Modulation (AM) simply modified for operation at 100 kHz.
AM broadcasting transmitters are designed to operate into constant impedance, typically 50 ohms. The concept of impedance implies a steady state relationship between the voltages and currents in the amplifiers and the antenna. With Loran, no such steady state relationship exists. The instantaneous impedance of the antenna (the ratio of voltage to current at one instant in time) varies throughout the pulse from a very large level initially to close to the steady state base impedance near the pulse peak, then decreasing and finally becoming negative during the pulse tail. When the impedance is negative, power is flowing out of the antenna back to the transmitter. The negative power flow necessitates the use of “tail biter” circuitry, currently in use in Loran transmitters.
Typical amplifiers are designed for operation when the load is thought of as a resistor meaning that the signal bandwidth is less than the antenna bandwidth and the induced current waveform directly follows the voltage waveform. AM broadcast transmitters use passive LC impedance matching and combining circuitry to match the antenna impedance to the optimum load impedance for the radio frequency amplifiers. As a result of the considerable change in the antenna impedance during the Loran pulse, passive LC impedance matching networks are not ideal for this situation.
Accordingly, there is a need to develop a more efficient amplifier that can be used to transmit a signal that is larger than the bandwidth of a narrow-band antenna.
It will be noted that throughout the appended drawings, like features are identified by like reference numerals.